Interferometric system with reduced vibration sensitivity and related method

ABSTRACT

A source module ( 12 ) generates mutually orthogonally polarized beams of light as emanating from two spatially separated point sources (Sv, Sw) for use in a phase shifting interferometer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional ApplicationSer. No. 60/429,669, filed Nov. 27, 2002, and U.S. ProvisionalApplication Ser. No. 60/459,149, filed Mar. 31, 2003, the contents ofboth which are incorporated herein by reference.

FIELD OF THE INVENTION

The instant invention is directed to an interferometric system andmethod, in particular, an interferometric system and related method forenabling measurements of a wavefront in the presence of vibration orother disturbances that impede accurate measurements.

BACKGROUND OF INVENTION

Interferometers have been known and used for a long time. They are usedfor many purposes, including measuring characteristics of gases,liquids, and materials, through the use of transmitted or reflectedlight. There exist many types of interferometers that are classified bytheir optical design. A few of the most widely used interferometer typesinclude Fizeau, Twyman-Green, Michaelson, and Mach-Zender. Each of theseoptical designs produces interference patterns called interferogramswhich are generated by the optical interference of test and referencewavefronts. In a typical interferometer, test and reference beams areobtained by appropriately splitting an incoming source beam (“beams” and“wavefronts” used interchangeably herein, with a “wavefront” beingunderstood by one of ordinary skill in the art as propagating along theoptical axis and sweeping out a volume that defines the light beam). Oneof the beams interacts with an object under test (hence commonlyreferred to as the “test beam”) thus carrying information about the testobject being measured, while the other interacts with a known referenceobject (hence, commonly referred to as the “reference beam”).Interfering or otherwise coherently superimposing these two wavefrontsproduces an interferogram.

Information about a measured object can be extracted from a singleinterferogram. This technique allows for fast data acquisition, however,it typically suffers from poor spatial resolution, time consuming andcomplex data processing and/or non-uniform data sampling. Thus, it isoften desirable to use other techniques instead. The most commontechniques use three or more phase-shifted interferograms (typicallythree to twelve). Using multiple phase-shifted interferograms providesadditional information that can be used to greatly increase the accuracyof the analysis.

Phase-shifting is a method used to change the phase between the test andreference wavefronts in a controllable way. During the last 20 years,various methods have been used to practically implement phase shiftingtechniques, including mechanically moving the reference object smalldistances comparable to the wavelength of light, or placingphoto-elastic modulators and crystal retarders in the beam path. Almostall of these methods use a sequential approach (serial in time) togenerate phase-shifted interferograms, which is accomplished byintroducing prescribed changes to the wavefront phase while a detectoracquires a series of data images. For example, the sequence of acquiringtemporal phase-shifted interferograms occurs as follows: acquireinterferogram, then shift the phase, acquire interferogram, then shiftthe phase, and so on. However, these known time-dependent methods aresensitive to environmental conditions during the span of time in whichseries of interferograms are acquired. Environmental conditions that canintroduce errors include vibration, airflow, temperature changes, objectmovements, etc. Vibration is usually the major cause of error. Elaboratemounts or expensive vibration isolation tables are commonly used toisolate temporal phase-shifted interferometers from the physicalenvironment.

To enable interferometric measurements under normal environmentalconditions, without special isolation equipment, instruments have beendeveloped to acquire multiple phase-shifted interferogramssimultaneously. This eliminates or greatly reduces the effect of theseerrors on measurements. However, such simultaneous phase shiftingmethods have to date been limited to particular types ofinterferometers, such as the Twyman-Green or Mach-Zender types discussedbelow.

U.S. Pat. No. 4,583,855 (issued to Barekat) entitled “Optical PhaseMeasuring Apparatus” relates to use of a polarization type Twyman-Greeninterferometer with quarter-waveplates and polarizers.(“Quarter-waveplates” and “half-waveplates” used herein are understoodby one of ordinary skill in the art as equivalent to quarter-waveretarders and half-wave retarders, respectively). Koliopulos in a paperentitled “Simultaneous Phase Shift Interferometer”, Proc. SPIE Vol.1531, p. 119 (1992), described the use of a polarization typeTwyman-Green interferometer. A. Hettwer, J. Krantz and J. Schwider in apaper titled “Three Channel Phase-Shifting Interferometer UsingPolarization Optics and A Diffraction Grating” Opt. Eng., 39(4) (April2000) described a Twyman-Green interferometer. German Patent DE196,52,113,A1 awarded to J. Schwider discloses the invention that isdescribed in his above-cited paper, based on a Twyman-Greeninterferometer. U.S. Pat. No. 6,304,330 entitled “Method and Apparatusfor Splitting, Imaging and Measuring Wavefronts in Interferometry” andU.S. Pat. No. 6,552,808 are directed to a modified polarization typeMach-Zender and Twyman-Green interferometers.

As intimated above, optical interferometers are typically constructed ofoptical components such as lenses, mirrors, beamsplitters, andwaveplates. These components usually have slight imperfections ordeviations from an ideal perfect component. From a practical standpoint,Twyman-Green type interferometers can suffer from a configuration havinga reference arm and a test arm that are of separate paths. Because theinterferogram generated by the interferometer is an image or patternthat registers differences between the test and reference wavefront, aseparation of the test and reference path such as in a Twyman-Green typeinterferometer, can cause imperfections and aberrations in the opticalcomponents encountered in one path, but not in the other path, toregister as measurement errors. That is, where the beam paths areseparate, an error in one path not present in the other path canregister in the final comparison result (the interferogram). Because theaforementioned interferometers have Twyman Green type configurations,they are susceptible to the disadvantages of separate paths between thetest and reference beams.

A well recognized advantage of a Fizeau interferometer is the feature ofa common path shared by the test and reference wavefronts throughoutmost of the interferometer. Where the test and reference wavefronts bothtravel through the same optical components, imperfections andaberrations in components are common to both wavefronts, and do notregister as measurement errors in the interferogram. Thus imperfectcomponents do not impart “difference errors” in the final comparison ofthe test object to the reference object. As such, the Fizeauconfiguration is significantly more tolerant and robust compared toother interferometry systems. Imperfect components in its constructionhave little or no effect on the accuracy and precision of the finalmeasurement results. This and other typical features of the Fizeau,including an alignment mode, ability to measure large flat optics, zoomcapabilities, and ease of use with corrective null optics, have made theFizeau a very popular, if not the most popular, interferometerconfiguration for practical applications.

However, despite such advantages of the Fizeau-type interferometers,there has been little, if any, ability or method known to construct oruse a Fizeau interferometer that is capable of simultaneousphase-shifting.

Accordingly, there is a desire for a Fizeau-type interferometer capableof simultaneous phase shifting, and, further, for a simultaneous phaseshifting Fizeau-type interferometer that uses orthogonally polarizedbeams.

SUMMARY OF THE INVENTION

The instant invention is directed to an interferometric system having asource module, an interferometry module and a simultaneous phaseshifting module. In particular, the source module generates mutuallyorthogonally polarized beams of light that are received by theinterferometry module for interaction with a reference object and a testobject. The interferometry module is configured with various opticalelements that define a common beam pathway so as to minimize theintroduction of measurement errors. Test and reference beams exiting theinterferometry module then enter the simultaneous phase shifting modulewhere at least two phase shifted interferograms are generatedsubstantially simultaneously.

More specifically, the present invention is directed to aninterferometric system, having a source module with a source ofpolarized light, a polarization beamsplitter element configured to acton the polarized light to generate mutually orthogonally polarized beamsof light, an interferometry module that includes a mechanism foroverlapping a test beam and a reference beam, and a phase shiftingmodule that generates at least two phase-shifted interferogramssubstantially simultaneously from overlapping test and reference beams.

The present invention may further provide a source module having apolarization beamsplitter element configured to generate mutuallyorthogonally polarized beams as emanating from two spatially separatedpoint sources (either real or virtual). The present invention alsocontemplates an interferometry module having a test object and areference, a beam splitter and a collimator, where the beamsplitter andthe collimator define a substantially common path for the twoorthogonally polarized beams, and the mechanism for overlapping permitsa selection of a specific pair of mutually orthogonally polarizedreference and test beams for processing by the simultaneousphase-shifting module.

The present invention specifically contemplates an interferometricsystem with a Fizeau or Fizeau-type front end assembly that processesorthogonally polarized test and reference wavefronts for input to asimultaneous phase-shifting module for purposes of generating two ormore phase-shifted interferograms, where the phase shifting may beaccomplished by a variety of simultaneous phase shifting methods. Thesimultaneous acquisition of multiple wavefronts results in robustmeasurements in the presence of vibration and other environmentalconditions.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments, taken in conjunction with theaccompanying drawings, which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of an embodiment of the present invention using apolarization beamsplitter element;

FIGS. 2A-2J are plan views of different embodiments of the polarizationbeamsplitter element of FIG. 1;

FIGS. 3A and 3B are plan views of an image displayed on an alignmentcamera of the invention of FIG. 1, showing, respectively, wavefrontswithout overlap before alignment, and wavefronts with overlap afteralignment; and

FIG. 4 is a plan view of another embodiment of the present inventionusing a quarter waveplate between a reference object and a test object.

DESCRIPTION OF THE INVENTION

An interferometric system 10 of the present invention is shown inFIG. 1. The illustrated embodiment of the system 10 has, in opticsparlance, a front end or front end assembly 11 and a back end or backend assembly 13. The front end 11 includes at least a source or sourcemodule 12 and an interferometry module 14. The back end 13 includes atleast a simultaneous (or substantially simultaneous) phase-shiftingmodule 20 for generating multiple phase-shifted interferograms suitablefor a wide variety of applications in many different fields. Someexamples include, without limitation, generating 3-D surface profiles,computing aberrations for tested optical systems, distribution ofvelocity of a gas flow chamber and distribution of refractive indexwithin optical materials. The embodiment of the back end 13 shown inFIG. 1 also includes an alignment module 16 and an imaging system 18. Inaccordance with the present invention, the source 12 is configured togenerate mutually orthogonally polarized beams that enter theinterferometry module 14 for purposes of interacting with a test opticor object (and a reference optic or object) whose characteristics are tobe acquired. That is, the characteristics of the test and referenceobject are imparted, respectively, to test beams and reference beamsemerging from the interferometry module 14. Advantageously, theinterferometry module 14 is configured as a Fizeau or a Fizeau-typecharacterized by a substantially common optical path for both referenceand test beams, between at least the non-polarizing beamsplitter 40 andreference surface Ra.

In the embodiment of FIG. 1, the reference and test beams emerging fromthe interferometry module 14 encounter the alignment module 16 and theimaging system 18 before entering the phase shifting module 20, which ispreferably a simultaneous phase shifting module, such as the module thatis subject of a co-pending application entitled “SIMULTANEOUS PHASESHIFTING MODULE FOR USE IN INTERFEROMETRY,” the contents of which areincorporated herein by reference. However, it is understood by one ofordinary skill in the art that any simultaneous phase shifting modulecapable of processing mutually orthogonally polarized beams may be usedwith the present invention. In accordance with the present invention,measurement results are complied from two or more (preferably three tosix) interferograms obtained simultaneously by the module 20.

The embodiment of the source 12 as shown in FIG. 1 has a polarizedsource 24 generating a beam of linearly polarized light (or wavefront) Bthat passes through a lens 28 which focuses the light through anaperture or pinhole 30 configured in a spatial filter 36. The wavefrontB then travels through polarization beamsplitter element 38 whichgenerates (or otherwise splits the wavefront B into) two mutuallyorthogonally polarized wavefronts V and W. The optical element 38 whichoperates as a polarization beamsplitter on the light beam B to producethe mutually orthogonally polarized wavefronts V and W can assume anumber of different configurations and/or embodiments not limited tothose discussed in detail further below.

In accordance with the present invention, the two mutually orthogonallypolarized wavefronts V and W exiting the polarization beamsplitterelement 38 are displaced with respect to each other as if theyoriginated from two slightly spatially separated (virtual or real)sources Sv and Sw, respectively. With respect to the embodiment of FIG.1 and for ease of discussion, the sources Sv and Sw are horizontallydisplaced from each other. That is, using the Cartesian coordinatesystem X-Y shown in FIG. 1, where the X axis is in the plane of thedrawing and the Y axis is perpendicular out of the plane, the sources Svand Sw have the same Y coordinate, but have different X coordinates.

Entering the interferometry module 14, the two wavefronts V and W(mutually orthogonally polarized and emanating from spatially separatedsources Sv and Sw, respectively) travel through various optics,including a non-polarizing beamsplitter 40, a quarter waveplate 42, acollimator 44 (whose focal plane defines the location of the virtualsources Sv and Sw), before they encounter a reference or known object R.There, a percentage of each of the two wavefronts V and W reflects off asurface Ra of the reference object R, while another percentage of thewavefronts V and W travels (to the left) toward a test object T. Thepercentage reflected off the surface Ra forms reference wavefronts Vrand Wr which (traveling to the right in FIG. 1) transmit back throughthe collimator 44 and the quarter waveplate 42 and reflect off thenon-polarizing beamsplitter 40 to exit the interferometry module 14. Assuch, the reference wavefronts Vr and Wr now carry characteristics orinformation about the reference surface Ra which were imparted to thesewavefronts as they reflected off or otherwise interacted with thereference surface.

The other percentage of the two wavefronts V and W that transmittedcompletely through the reference object R continues to travel toward thetest object T (to the left in FIG. 1). A reflection off the test objectT forms test wavefronts Vt and Wt (traveling to the right in FIG. 1)which then return through the reference object R, the collimator 44 andthe quarter waveplate 42 before reflecting off the non-polarizingbeamsplitter 40 to exit the interferometry module 14. The testwavefronts Vt and Wt now carry characteristics or information about thetest object T which were imparted to these wavefronts as they reflectedoff or otherwise interacted with the test object T. It is understood byone of ordinary skill in the art that depending on the opticalproperties of the test object T, wavefronts incidental on the testobject T can also can transmit through the test object T and reflect offa second reference object R′ (to create Vt′ and Wt′, not shown). In thelatter event, the wavefronts Vt′ and Wt′ are treated by the system 10 ina fashion similar to that described herein for the wavefronts Vt and Wt.

It is understood by one of ordinary skill in the art that the collimator44 can be obviated from the module 14 where the reference object R isconfigured with appropriate surface curvature to direct or focus thewavefronts Vt, Wt (or Vt′ or Wt′) back along the same path traveled bythe wavefronts V and W entering the object R. It is further understoodby one of ordinary skill in the art that the quarter waveplate 42 is anoptional component of the interferometry module 14 and is commonly usedto produce circularly polarized light which is often preferred formeasurements.

In the embodiment of FIG. 1, a portion of the four wavefronts Vr, Wr, Vtand Wt exiting the interferometry module 14 are diverted to thealignment module 16, by reflection off a mirror 50 toward an alignmentcamera 52. The mirror 50 is positioned or flipped out of the beam pathwhen the system 10 is operating in the measurement mode, and positionedor flipped back in the beam path during the alignment mode. It isunderstood by one of ordinary skill in the art that the alignment module16 is provided primarily for the user's convenience and is not anecessary component of the present invention for purposes of generatingeffective interferograms. When used, the alignment camera 52 ispositioned at the focal point of the portion of the wavefronts Vr, Wr,Vt and Wt reflected off the mirror 50 so that each of these reflectedportions of the wavefronts forms a localized image or spot on an imagesensor of the camera 52.

As shown in FIG. 3A, an image 70 of the camera 52 displays a pluralityof four localized images or spots, each of which corresponds to one ofthe wavefronts Vr, Wr, Vt and Wt. The relative positioning and pluralityof the spots, namely four, are due to the spatial separation between thesources Sv and Sw, the angular tilt position of the reference object Rand the test object T (or the reference object R′, as the case may be).In particular, the x displacement between the dots of the wavefronts Vrand Wr (or Vt and Wt) corresponds with the x displacement between thevirtual sources Sv and Sw (see FIG. 1) and the displacement along the yaxis between the dots of the wavefronts Vr and Vt (or Wr and Wt)corresponds to the relative tilt orientation of (or angle between) thereference object R and the test object T (or the reference object R′, asthe case may be).

In order to generate an interferogram purposeful for revealinginformation about the test object T, a test wavefront is to at leastoverlap a reference wavefront. Consequently, orthogonally polarizedwavefronts are to overlap sufficiently at the input of the simultaneousphase-shifting module 20, in order for simultaneous phase-shiftedinterferograms to be generated. Accordingly, of the four polarizedwavefront spots, either the orthogonal pair Vr and Wt are to overlap, orthe orthogonal pair Wr and Vt are to overlap. To that end, the alignmentcamera 52 provides the user with a view of the relative positioning ofthe four wavefronts and any visible degree of overlap between them.

In the situation shown in FIG. 3A, the four spots of the wavefronts Vr,Wr, Vt and Wt of image 70 are without any visible degree of overlap. Inthat regard, the reference object R and the test object T are mounted ontip-tilt mechanisms, as understood by one of ordinary skill in the art,to enable the user to adjust the relative positioning or orientationangle of the objects R and T so as to manipulate the four spots into anoverlapping position or relationship on the image 70. By tipping and/ortilting either the reference object R or the test object T, the user canmove and reposition the spots so that the pair of the wavefronts Wr andVt are superimposed, or that the pair of the wavefronts Wt and Vr aresuperimposed.

As shown in the FIG. 3B, the user has adjusted the tip-tilt mechanismsof the interferometry module 14 such that the image 70 indicates anoverlap between the spots of the wavefronts Wr and Vt. The remaining twospots Vr and Wt in FIG. 3B are separated, and their spacing is such thatthey will not pass through the aperture hole 54 in the spatial filter56. The spatial separation of Sv and Sw and the size of the aperturehole 54 are selected such that when two orthogonal spot pairs (either Wrand Vt, or Wt and Vr) are overlapped, the remaining two spots areblocked by the spatial filter 56. If these blocked wavefronts wereallowed to pass the spatial filter 56, they would contribute undesirablecoherent background light in the module 20, resulting in noise in thefinal measurement result. (The size of the aperture hole 54, and thespatial separation of Sv and Sw can be constructed to be adjustable, sothey can be varied for special applications.)

The wavefronts Wr and Vt are now appropriately positioned relative toeach other as shown in FIG. 3B. Portions of the four wavefronts Vr, Wr,Vt and Wt have bypassed the mirror 50, and proceeded to enter theimaging system 18. Wavefronts Vr and Wt are blocked by the spatialfilter 56 (see FIG. 3B). The two overlapped wavefronts Wr and Vt passthrough the aperture hole 54 of the spatial filter 56, and transmitthrough collimator 58, before encountering a diffuser 60. It isunderstood by one of ordinary skill in the art that the user could haveselected the alternative the pair of the spots Vr and Wt by operatingthe tip-tilt mechanisms accordingly.

Because the diffuser 60 maintains the polarization, the overlappedorthogonal wavefronts Wr and Vt, which form a disc of light 62 on thediffuser, will remain orthogonally polarized as they propagate beyondthe diffuser 60. It is understood by one of ordinary skill in the artthat the diffuser 60 is optional and that it is used to reduce specklein the resulting interferograms. That is, the diffuser 60 can bedesirable, but is not a necessary component of the present invention forthe purpose of simultaneously sets of phase-shifted interferograms. Inany case, the wavefronts Wr and Vt forming the disc of light 62 on thediffuser 60 are then imaged or otherwise relayed by lenses 64 (e.g.,zoom lenses) to the simultaneous phase-shifting apparatus 20, with theirmutually orthogonal polarizations maintained in the state they were inon the surface of the diffuser 60. The wavefronts Wr and Vt can now bemanipulated and processed by the module 20 to interfere and produceinterferograms, of which two or more (preferably three to six)phase-shifted interferograms may be produced substantiallysimultaneously and used for final analysis.

FIGS. 2A-2J show various examples of embodiments of the polarizationbeamsplitter element 38 of the source 12. In particular, FIGS. 2A and 2Billustrates a Wollaston prism. FIGS. 2C and 2D illustrate a singlecalcite beam displaces. FIG. 2E illustrates dual calcite beam displacerswith a half waveplate. FIG. 2F illustrates dual fiber optics. FIGS. 2Gand 2H illustrate a fiber optic splitter. FIG. 2I illustrates apolarizing lateral displacement beamsplitter and FIG. 2J illustrates apolarizing cube beamsplitter and mirror.

In accordance with an aspect of the present invention, the polarizationtype beamsplitter element 38 functions to produce the two mutuallyorthogonally polarized beams V and W, and further to produce such beamsas originating from spatially separated sources (virtual or real). Asunderstood by one of ordinary skill in the art, the possible embodimentsof the polarization beamsplitter element 38 with the aforementionedfunctions is not limited to the embodiments discussed in detail below.

Referring to FIG. 2A, a prism 90 of the Wollaston type is shownpositioned after the focusing lens 28 (and after the spatial filter 36shown in FIG. 1, but not show here). The focusing lens 28 focusesincoming polarized light B (from a source, not shown) to its focal point94 from which the light diverges and enters the prism 90 which ispositioned beyond the focal point 94. The prism 90 acts on the polarizedlight and splits it into two orthogonally linearly polarized beams V andW that are angularly displaced with respect to each other by a smallangle, thereby creating the virtual sources Sv and Sw. This angle isdefined by the geometry of the prism 90 and its birefringent materialfrom which it is made. By specifying the type of material and geometryof the prism, it is possible, as understood by one of ordinary skill inthe art, to control the separation of the virtual sources Sv and Sw inthe focal plane of the collimator 44 (FIG. 1). This embodiment isdesirable for its simplicity, but the use of a diverging beam incidentalon the prism may introduce aberrations including astigmatism thatnormally should not be present in the illuminating beams V and W,because it would typically affect the measurements results. Theseaberrations can be compensated for with optics (or calibrated out of themeasurements) as understood by one of ordinary skill in the art.

Referring to FIG. 2B, the prism 90 of this embodiment is positioned inthe collimated beam B after the source 24. To eliminate astigmatism fromthe illuminating beams V and W, which can arise from the prism 90 asdiscussed above, the prism 90 is placed in the polarized collimated beambefore the focusing element 28. The narrow collimated beam after passingthrough the Wollaston prism 90 becomes two orthogonally polarizedcollimated beams that are angularly displaced with respect to each otherby a small angle which is defined by the geometry of the prism andproperties of the birefringent material from which the prism is made.After passing through the prism 90, the two angularly separated beamspass through the focusing lens 28, and are focused to two separatepoints Sv and Sw. The separation distance between the points isdetermined by the angular separation of the beams in the prism 90 andthe focal length of the focusing lens 28. With this configuration, theresulting beams V and W are generally free from astigmatism. Moreover,the prism 90 can be of a relatively smaller size since it is used with acollimated beam, and the direction and divergence of the two beams canbe better controlled. However, if the beams are to be spatiallyfiltered, two spatial filter pinholes 136 and 136 b are used along withthe associated mechanisms for placement and adjustment of two pinholesin the focal plane of the focusing lens 28. Furthermore, better opticalaberration correction of the focusing lens 28 may be appropriate due tothe difference in incident angle of the illuminating beams on lens 28.

The embodiment of FIG. 2C uses a calcite beam displacer 100 positionedafter the focusing lens 28. The calcite beam displacer 100 is a singleblock of calcite (or other birefringent material with similar beamdisplacing effect), cut with the proper orientation of its fast axis.The beam displacer 100 is placed after the focal plane of the focusinglens 28 and pinhole 30. The calcite block 100 separates properlyoriented linearly polarized diverging beam 102 into two orthogonallypolarized wavefronts V and W (dotted ray tracing representing theoptical axis of the diverging beam with a polarization vectorperpendicular to the plane of the drawing and solid ray tracingrepresenting the optical axis of the diverging beam with a polarizationvector parallel with the plane of the drawing) that are laterallydisplaced from each other. The two beams after passing through thecalcite block 100 create two virtual sources Sv and Sw in the focalplane of the collimator 44 (see FIG. 1). The separation between thesources is a function of the designed length of the calcite crystal 100.However, the diverging beam 102 upon passing through the calcite crystal100 acquires astigmatism that typically affects the measurement results.Again, as understood by one of ordinary skill in the art, theseaberrations can be compensated for with optics (or calibrated out of themeasurements).

Referring to FIG. 2D, the calcite beam displacer 100 of this embodimentis placed in the collimated beam after the source 24. That is, in orderto eliminate astigmatism from the illuminating beams V and W when usingthe single calcite beam displacer 100, it is placed in the polarizedcollimated beam path before the focusing elements 28 a and 28 b. Thenarrow collimated beam after passing through the beam displacer 100 willseparate into two orthogonally polarized collimated beams that arelaterally displaced with respect to each other. After the beam displacer100, the two parallel beams then pass through two separate focusinglenses 28 a and 28 b, and are focused to two separate points Sv and Sw.The separation distance between the two points is a function of thedesigned length of the calcite crystal 100. As mentioned above, theresulting beams of this configuration are generally free fromastigmatism, and the beam displacer can be made smaller. However, twolenses are needed and they are required to be nearly identical, so asnot to introduce different aberrations in the test and reference beams.Two spatial filter pinholes 137 a and 137 b would be used to spatiallyfilter the beams, along with associated mechanisms for placement andadjustment of two pinholes in the focal plane of the focusing lens 28 aand 28 b.

Referring to FIG. 2E, dual calcite beam displacers 100 a and 100 b and ahalf waveplate 106 are used in the illustrated embodiment, which may bea preferred embodiment of the polarization beamsplitter element 38 ofthe present invention. This configuration or assembly is located afterthe focal plane of the focusing lens 28 and the pinhole 30. The secondbeam displacer 100 b is selected to have the same (effective) length asthe first beam displacer 100 a, but with a rotation of 180° about itsoptical axis. The half-wave-plate 106, or other polarization rotationdevice, is placed between the two beam displacers 100 a and 100 boriented with the fast axis at 45° with respect to both of the linearlypolarized beams. With this orientation, the half waveplate 106 rotatesthe polarization directions of both incoming beams by 90°, therebyenabling the assembly to cancel out the astigmatism that would bepresent with either block 100 a or 100 b acting alone. It is furtherunderstood by one of ordinary skill in the art that the length of thecalcite beam displacers 100 a and 100 b can be customized to control thespacing between virtual sources Sv and Sw that are produced.

The embodiment illustrated in FIG. 2F uses a polarizing cubebeamsplitter 108 (or other device that separates the polarizations in asimilar way), which splits the source beam 102 into two orthogonallypolarized wavefronts V and W that are coupled into proximal ends of twopolarization preserving optical fibers 112 and 114. The distal ends ofthe fibers 112 and 114, the outputs, are positioned proximately to eachother, but with a spatial separation, in the focal plane of thecollimator 44. Light leaves the outputs of the fibers 112 and 114 asmutually orthogonally polarized wavefronts V and W. Advantageously, thefibers 112 and 114 by their structure and configuration obviate the needfor spatial filter pinholes and readily enable adjustment of the spatialseparation between the sources Sv and Sw.

Referring to FIG. 2G, the illustrated embodiment has a polarizationmaintaining y-coupling fiber 120 to split the polarized source beam 102into two wavefronts in fibers 122 and 124. One of the wavefronts (thewavefront traveling in the fiber 124 in the case of FIG. 2G) is thenorthogonally polarized with respect to the other wavefront (thewavefront traveling in the fiber 122 in the case of FIG. 2G) by aninline polarization rotation device 126. The resulting orthogonallypolarized wavefronts V and W are outputted from the y-coupling fiber120.

Referring to FIG. 2H, the illustrated embodiment also has thepolarization maintaining y-coupling fiber 120 to split the polarizedsource beam 102 into two wavefronts in the fibers 122 and 124. However,a half waveplate 130 or other similar or equivalent device is providedto rotate the polarization of the wavefront exiting the fiber 126 withrespect to the wavefront exiting the fiber 122.

It is understood by one of ordinary skill in the art that for each ofthe fiber optic methods above of FIGS. 2G and 2H, the two fiber optics122 and 126 may be configured to allow wavefronts V and W to be spacedadjacent (with a relative spatial displacement between Sv and Sw) andparallel as shown in FIG. 2F or adjacent and adjust angularly as shownin FIGS. 2G and 2H as desirable or appropriate. It is further understoodby one of ordinary skill in the art that the system use any of thefollowing to provide orthogonally polarized output wavefronts V and W: apolarization splitter based on photonic crystal fibers, a fiber opticPolarization Beam Splitter/Combiner with polarization maintaining fiberpigtails, or an integrated waveguide polarization splitter.

The embodiments illustrated in FIG. 2I and FIG. 2J represent twovariations of a lateral displacement beamsplitter. The lateraldisplacement beamsplitter 140 placed after the pinhole 36 splits theincoming beam 102 into two orthogonal polarized beams V and W,originating from virtual sources Sv and Sw. The separation between thesources is defined by the geometrical size of the beamsplitter. Thepolarizing cube beamsplitter 141 with mirror 142 (FIG. 2J) is avariation of the lateral displacement beamsplitter 140. The two willhave similar performance.

Referring to FIG. 4, yet another alternative embodiment of the system 10of the present invention is shown, having a quarter waveplate 45 in lieuof both the polarization beamsplitter element 38 and quarter waveplate42 (although the system will still function with the quarter waveplate42 in place). The optical path of the system 10 as substantially as thatdescribed in reference to the embodiment of FIG. 1, although thetreatment and manipulation of the beam traveling the optical pathdiffers from that of the embodiment of FIG. 1 However, despite thesedifferences, the wavefronts that exit the interferometry module 14 ofthe embodiment of FIG. 4 are nevertheless mutually orthogonallypolarized in accordance with the present invention.

In the embodiment of FIG. 4, the polarized source 24 generates thepolarized beam of light (or wavefront) B that passes through the lens28, which focuses the light through an aperture or pinhole 30 configuredin a spatial filter 36. The wavefront B, as emanating from a singlepolarized source S, then enters the interferometry module 14 where itpasses through the non-polarizing beamsplitter 40, the collimator 44,and counters the reference or known object R. There, a percentage of thewavefront B reflects off a surface Ra of the reference object R, whileanother percentage of the wavefront B travels toward the quarterwaveplate 45. The percentage reflected off the surface Ra formsreference wavefront Br which (traveling to the right in FIG. 4)transmits back through the collimator 44 and reflects off thenon-polarizing beamsplitter 40 to exit the interferometry module 14. Assuch, the reference wavefront Br now carries characteristics orinformation about the reference object R which were imparted to thiswavefronts as it reflected off or otherwise interacted with thereference object.

The other percentage of the wavefront B that is transmitted completelythrough the reference object R continues to travel toward the quarterwaveplate 45 where it is converted to circular polarization beforereflecting off the test object T and forms test wavefront Bt (travelingto the right in FIG. 1). Then it again counters the quarter waveplate45. It is understood by one of ordinary skill in the art that thequarter wave plate 45 could always be oriented in such a way that thebeam Bt after passing through the quarter wave plate 45 will beorthogonally polarized with respect to the reference beam Br. Thewavefront Bt then continues through the reference object R and thecollimator 44 before reflecting off the non-polarizing beamsplitter 40to exit the interferometry module 14.

The wavefronts Br and Bt now carry characteristics or information aboutthe test object T which were imparted to these wavefronts as theyreflected off or otherwise interacted with the test object T, and aremutually orthogonally polarized before entering the simultaneous phaseshifting module 20 for processing to produce interferograms suitable.Again, it is understood by one of ordinary skill in the art thatdepending on the optical properties of the test object T, the otherpercentage incidental on the test object T can also can transmit throughthe test object T and reflect off a second reference object R′ (tocreate Br′, not shown). In the latter event, the wavefront Br′ istreated by the system 10 in a fashion similar to that described hereinfor the wavefronts Vt and Wt.

It is understood by one of ordinary skill in the art that theinterferometer described in the present invention can be used as astandard phase shifting Fizeau-type interferometer providing that astandard phase shifting mechanism is present. Additionally, because thesystem of the present invention produces and uses orthogonally polarizedtest and reference beams (which can be in the visible light spectrum orother regions of the electromagnetic spectrum with longer or shorterwavelengths), it is possible to use a variable phase retarder after thesource 12 to induce phase shifts. This would normally alleviate the needin a standard Fizeau to phase shift by physically moving the referenceelement, which can be large for testing large optics.

Additionally, an important aspect of the present invention allows for avariable intensity ratio between reference and test beams by rotatingthe polarization of the source 24. This would normally allow formeasurements of a variety of objects with different coefficients ofreflection (or transmission) without the use of an attenuator. Thepolarization from the source 24 can be rotated by physically rotatingthe source, or by optically rotating the polarization of the source.Where the source is linearly polarized, the polarization can be rotatedby inserting a half waveplate after the source 24 and adjusting itsrotation. This would normally change the amount of intensity in the twoorthogonally polarized beams W and V, making one brighter than theother. If the test object is relatively more reflective, then it wouldtypically be advantageous to decrease the intensity delivered to thetest object so the reflected beam's intensity is roughly equal to thebeam reflected from the reference object. This produces fringes ofhigher contrast in the interferograms.

In yet another embodiment of the present invention (referring to FIG.1.), the laser 24 is replaced by an appropriate multi-wavelength source,or multi-wavelength source assembly for dual wavelength interferometry.Examples of these types of sources include a source with a broad enoughbandwidth such that select wavelengths can be filtered out for use(either simultaneously or temporally for individual simultaneousmeasurements at each wavelength), a tunable laser, at least two separatesources that are combined so that their beams are substantiallycoincident, and multiple sources coupled in to a fiber or fibers.

Optical components of interferometer front-end and back-end would bemodified if necessary to provide achromatic properties. Phase-shiftingmodule 20 would then be replaced with a phase-shifting module capable ofprocessing multiple wavelengths for dual-wavelength interferometry.Among other applications, this would increase the dynamic range orheight measuring capabilities of the invention, when measuring 3Dprofiles.

It is further understood by one of ordinary skill in the art that anysimultaneous phase shifting apparatus that uses orthogonally polarizedtest and reference beams at its input, can be used in lieu of the module20, to produce multiple interferograms.

It is also further understood by one of ordinary skill in the art thatother types of interferometers, common path interferometers, anddifferential interferometers, can be adapted in a similar way (as theclassical Fizeau-type was here), to be converted to a simultaneous phaseshifting configuration.

It is understood by one of ordinary skill in the art that the scope ofthe invention is not limited to the embodiments described above. Manyother modifications and variations will be apparent to those of ordinaryskill in the art, and it is therefore, to be understood that within thescope of the appended claims the invention may be practiced otherwisethan as specifically described.

1. (canceled)
 2. An interferometric system of claim 35, wherein saidinterferometric module is configured to define a substantially commonpath for said beams between said source module and a reflective surfaceof said reference object.
 3. (canceled)
 4. An interferometric system ofclaim 35, wherein said reference beam emanated from one of saidspatially separated sources and said test beam emanated from another ofsaid spatially separated sources.
 5. An interferometric system of claim35, wherein said reference and test beams received by said simultaneousphase shifting module substantially overlap each other.
 6. Aninterferometric system of claim 35, wherein the mutually orthogonallypolarized beams are coherent.
 7. An interferometric system of claim 35,wherein there are two of said spatially separated sources.
 8. Aninterferometric system of claim 35, further
 9. (canceled)
 10. (canceled)11. An interferometric system of claim 35, wherein said sources arevirtual.
 12. An interferometric system of claim 35, wherein said sourcesare real.
 13. An interferometric system of claim 35, wherein theinterferometry module further includes a nonpolarizing beamsplitter. 14.An interferometric system of claim 13, wherein the nonpolarizingbeamsplitter is positioned substantially between the source module andthe reference object.
 15. An interferometric system of claim 35, whereinthe interferometry module further includes a quarter waveplatepositioned between the source module and the reference object.
 16. Aninterferometric system of claim 15, wherein the quarter waveplate ispositioned substantially between the nonpolarizing beamsplitter and acollimator.
 17. An interferometric system of claim 35, wherein theinterferometry module is of a Fizeau configuration.
 18. Aninterferometric system of claim 8, wherein the alignment module ispositioned to intercept the beams between the interferometry module andthe simultaneous phase-shifting module.
 19. An interferometric system ofclaim 9, wherein the imaging module is positioned to intercept the beamsbetween the interferometry module and the simultaneous
 20. (canceled)21. An interferometric system of claim 35, wherein said polarizationbeamsplitter comprises a prism.
 22. An interferometric system of claim35, wherein said polarization beamsplitter comprises a calcite beamdisplacer.
 23. An interferometric system of claim 35, wherein saidpolarization beamsplitter comprises two calcite beam displacers and ahalf waveplate.
 24. An interferometric system of claim 35, wherein thepolarization beamsplitter comprises two fiber optics and cube polarizingbeamsplitter.
 25. An interferometric system of claim 35, wherein thepolarization beamsplitter comprises a polarizing lateral displacementbeamsplitter.
 26. An interferometric system of claim 35, wherein thepolarization beamsplitter comprises a cube polarizing beamsplitter andmirror.
 27. An interferometric system of claim 35, further comprising afilter to block said other portion of the beams from entering thesimultaneous phase shifting module.
 28. An interferometric system ofclaim 27, wherein said filter is configured with an aperture to permitpassage of said portion of the beams received by the simultaneous phaseshifting module. 29-34. (canceled)
 35. An interferometric system,comprising: a source module having a source of polarized light and apolarization beamsplitter configured to act on said polarized light togenerate mutually orthogonally polarized beams of light; aninterferometry module receiving said orthogonally polarized beams fromsaid source, having optical elements, a reference object and a testobject, said interferometry module further comprising a mechanism formanipulating a test beam and a reference beam into an overlappingposition; a phase shifting module receiving a portion of said beams fromsaid interferometry module to generate at least two phase-shiftedinterferograms substantially simultaneously from said test and referencebeams, and an alignment camera which provides a view of relativepositioning of the wavelengths and degree of overlap between them. 36.An interferometric system of claim 35, wherein said polarized light fromsaid source module is linearly polarized.
 37. An interferometric systemof claim 35, wherein the mechanism for manipulating comprises a tip-tiltmechanism.
 38. An interferometric system, comprising: a source modulehaving a source of linearly polarized light, and a polarizationbeamsplitter configured to generate mutually orthogonally polarizedwavefronts as emanating from two spatially separated sources; aninterferometry module receiving said orthogonally polarized wavefronts,said interferometry module having a test object and a reference, a beamsplitter and a collimator, wherein orthogonally polarized referencewavefronts and orthogonally polarized test wavefronts exit theinterferometry module; a tip-tilt mechanism for overlapping one of saidorthogonally polarized reference wavefront with one of said orthogonallypolarized test wavefronts; a simultaneous phase shifting modulereceiving said overlapping one reference wavefront and said one testwavefront from said interferometry module for generating at least twophase-shifted interferograms substantially simultaneously, wherein saidwavefronts follow a substantially common path through saidinterferometric system.
 39. An interferometric system of claim 38,wherein said portion of said beams comprises mutually orthogonallypolarized reference and test beams.
 40. An interferometric system ofclaim 39, wherein said reference beam emanated from one of saidspatially separated sources and said test beam emanated from another ofsaid spatially separated sources.
 41. An interferometric system of claim38, wherein the mutually orthogonally polarized beams are coherent. 42.An interferometric system of claim 38, wherein there are two of saidspatially separated sources.
 43. An interferometric system of claim 38,further comprising an alignment module.
 44. An interferometric system ofclaim 38, further comprising an imaging module.
 45. An interferometricsystem of claim 38, wherein the source module includes a linearlypolarized light source and a polarization beamsplitter configured tosplit linearly polarized light into said two mutually orthogonallypolarized beams.
 46. An interferometric system of claim 38, wherein saidsources are virtual.
 47. An interferometric system of claim 38, whereinsaid sources are real.
 48. An interferometric system of claim 38,wherein the interferometry module further includes a nonpolarizingbeamsplitter.
 49. An interferometric system of claim 48, wherein thenonpolarizing beamsplitter is positioned substantially between thesource module and the reference object.
 50. An interferometric system ofclaim 38, wherein the interferometry module further includes a quarterwaveplate positioned between the source module and the reference object.51. An interferometric system of claim 50, wherein the quarter waveplateis positioned substantially between the nonpolarizing beamsplitter and acollimator.
 52. An interferometric system of claim 38, wherein theinterferometry module is of a Fizeau configuration.
 53. Aninterferometric system of claim 52, wherein the alignment module ispositioned to intercept the beams between the interferometry module andthe simultaneous phase-shifting module.
 54. An interferometric system ofclaim 44, wherein the imaging module is positioned to intercept thebeams between the interferometry module and the simultaneous phaseshifting module.
 55. An interferometric system of claim 38, wherein thesource module includes a polarization beamsplitter configured tointeract with a beam from a source to provide said mutually orthogonallypolarized beams.
 56. An interferometric system of claim 55, wherein saidpolarization beamsplitter comprises a prism.
 57. An interferometricsystem of claim 55, wherein said polarization beamsplitter comprises acalcite beam displacer.
 58. An interferometric system of claim 55,wherein said polarization beamsplitter comprises two calcite beamdisplacers and a half waveplate.
 59. An interferometric system of claim55, wherein the polarization beamsplitter comprises two fiber optics andcube polarizing beamsplitter.
 60. An interferometric system of claim 55,wherein the polarization beamsplitter comprises a polarizing lateraldisplacement beamsplitter.
 61. An interferometric system of claim 55,wherein the polarization beamsplitter comprises a cube polarizingbeamsplitter and mirror.
 62. An interferometric system of claim 38,further comprising a filter to block said other portion of the beamsfrom entering the simultaneous phase shifting module.
 63. Aninterferometric system of claim 62, wherein said filter is configuredwith an aperture to permit passage of said portion of the beams receivedby the simultaneous phase shifting module.